I have a 503cw and never regretted it. They are cheap, under 1300 Euro with a winder, film back, WLF. Most lenses, except the latest 40mm are cheap and easy to find used.
Try to find one for a few days and do some tests.
The Winder takes 6 AA batteries. If you ever run out of batteries, to the Winder off, and use the body like a 501.
The 503cw is lighter, yes, and can be carried around for a day, it's a beast in studio, and on location.
If you are not used to manual focus, it can be a bit tricky, but there are different viewfinders available. Mirror slap or camera shake is not an issue handheld if you work 1/125 or higher (with a film back i can use 1/60 easily).
Tom